Packaging material for enclosing liquid or flowable food products is typically provided in the form of a continuous laminate web. The packaging material has a core layer made of paper or carton that is covered by a polymeric layer on a first, or outer, side. On the other side (i.e. the inner side facing the product to be enclosed) a multi-layer sheet is provided, typically comprising a layer of polymeric material, a barrier film, and an additional layer of polymeric material.
The barrier film comprises a layer which first and foremost constitutes a barrier to oxygen. A preferred such layer consists of a metal foil layer, preferably an aluminium foil layer. Disposable packaging containers, particularly those for storing liquids, are frequently produced from a laminated packaging material which consists of a carcass layer of paper, which layer is covered with thermoplastic materials and aluminium foil. The laminated packaging material is frequently supplied in the form of webs which are reeled on storage reels and which, after having been unrolled from their storage reel and cut into the desired width, are converted, by means of folding and sealing, into packaging containers in automatic packaging machines.
Such conversion may be done by unrolling the web from the storage reel and then forming a tube by the edges of the web being joined in an overlap seam, after which the tube which has been formed is filled with the intended filling product and subdivided into individual packaging containers by means of repeated transverse seals, which are arranged at a distance from each other at right angles to the tube. After the filling product which has been supplied has been enclosed in this way in sealed-off parts of the tube, these parts are separated from the tube by means of cuts made in the transverse sealing zones. The subdivided tube parts are then shaped, by means of folding along crease lines which are arranged in the packaging material, to form packaging containers of the desired shape, e.g. a parallelepipedic shape.
Packaging containers of this type are frequently provided with opening notches in the form of holes, openings or slits which are made in the packaging material and which are covered with strips which can be torn off and which are normally termed “pull-tabs”. Alternatively the packaging device is provided with an external opening device, for example in the form of a plastic pouring spout having a screw top for resealing, which opening device is only allowed to penetrate the packaging laminate in connection with the packaging being opened and the product being used. In this connection, the packaging laminate is provided with an opening which consists of a punched-out hole in the core layer, leaving only the barrier film and the thermoplastic layers to cover the hole. Hence, the non-paper layers are extending throughout the laminated packaging material and thus cover the pre-punched holes in the core layer.
Thus, when the filling material consists of a sterile product, such as sterilized milk, or an acidic product, such as orange juice, the packaging container is frequently manufactured from a packaging laminate comprising an aluminium foil layer as the barrier film which makes the packaging impervious to the penetration of gases, such as oxygen, which can oxidize the contents and impair its quality. In order to achieve the desired imperviousness, it is important that the aluminium foil layer is not broken or damaged during the shaping of the packaging or when the packaging material is being manufactured and, for the function of the tear strip opening (the pull tab) or the penetrating opening device, it is of importance that the aluminium foil layer adheres well to the area around and within the opening holes since otherwise the opening operation can easily fail. Thus when a covering strip has been affixed over the intended opening, this strip can be torn off in connection with this operation, without, for all that, the inner lining of plastic and aluminium foil being torn open. When a penetrating opening device is used, this opening device can fail to make a clean cut in the aluminium foil and thermoplastic layer, resulting in frayed edges.
For this purpose, the packaging material is manufactured in a series of laminating steps. In a first step, the pre-punched core layer is provided with a layer of thermoplastic material and the barrier film is applied to the laminated side of the core layer and an outermost layer of thermoplastic material is laminated to the barrier layer. Preferably, the step of applying the layer of thermoplastic material to the core layer and the step of applying the barrier film may be done in a single step.
In this step, as well as in the final step, i.e. when the outermost layer of thermoplastic material is laminated to the barrier film, different problems may arise due to the provision of the punched-out holes. As the laminate of core material is supplied from a first line, and the barrier film and the outermost polymer layer are supplied from other lines, the core material, the barrier film and the outermost polymer layer will be laminated in a nip between an impression roller and a cooling roller rotating against each other. Typically, the impression roller has a core body made of a rigid material, and an outer surface being made of a less rigid material, i.e. the outer layer covering the core body is made of a more elastic material than the core body of the roller.
The pressing force of the nip between the impression roller and the cooling roller will force the outer surface of the impression roller to deform, such that the material of the outer surface of the impression roller is forced to move in the feeding direction, or machine direction, of the laminating station.
When the pre-punched hole of the core layer laminate enters the nip, the membrane covering the hole (i.e. the sandwich formed by a layer of thermoplastic material, the barrier film, and a further layer of thermoplastic material) will be more flexible and more able to stretch than the portions of the packaging material having the core layer of paper or carton. Hence, deformation of the outer surface of the impression roller may force the membrane covering the pre-punched hole to stretch and deform, leading to a tucking effect in the machine direction of the laminating station. During the last step of lamination, the barrier film and the thermoplastic layer will thus be rolled and tucked such that tensile stresses are assembled within the packaging material. This situation may cause the barrier film to break, wherein the barrier properties of the packaging material is lost or significantly reduced. This leads to an increase of waste material due to imperfect laminated holes.